I have developed a technique for isolating UV-sensitive mutants of cultured Chinese hamster cells. The method uses nylon cloth replica plating and a means of identifying UV-sensitive colonies by the formation of plaques in a cell monolayer. The replica plating technique enables 2 thousand colonies per plate (approximately 2 times 10 to the 5th power total) to be examined for sensitivity to a specific lethal agent, but still permits the isolation of viable cells from a particular colony. I propose to use this method to isolate a series of independent UV-sensitive mutants. Also, I propose to develop methods for isolating mutants of the Chinese hamster cell that are unusually sensitive to other known mutagenic and carcinogenic agents such as X-rays and chemical agents. Studies of these mutants will then be undertaken for the purpose of identifying the biochemical events that are important in chromosomal repair and the relation these processes may have to malignant transformation. This genetic and biochemical approach to DNA repair has been very successful in bacteria. Applying this approach to mammalian cells will offer a means of studying biochemical events whose importance may not yet be fully recognized. With a set of mutants sensitive to a particular agent, a genetic and biochemical analysis can then be made of the steps necessary for increased survival to that agent, and of the biochemical basis for repair of exogenously caused damage to DNA. The relation between these biochemical defects and those observed in cells from patients with the disease Xeroderma pigmentosum can be examined. Also, the correlation between these defects and the effect on mutagenicity and malignant transformation can be investigated.